The present invention is directed to a cladded optical fiber having at least one longitudinal side coupling zone which is intended for lateral input and output coupling of light into and out of the fiber which has a glass core surrounded by a glass layer forming the cladding layer which cladding layer has an index of refraction lower than the index of refraction of the core so that a jump or step in the index of refraction occurs at the boundary surface between the core and cladding layer.
Optical glass fibers having a core surrounded by a cladding are known and the provision of longitudinal side coupling zones in such a fiber is also known. In an article by T. Ozeki and B. S. Kawasaki, "Optical Directional Coupler Using Tapered Sections In Multi-Mode Fibers", Applied Physics Letters, Vol. 28, No. 9, May 1, 1976, ppg. 528-529, a coupling zone between a plurality of fibers was formed by a local constriction of the cladded optical fiber so that the core diameter and casing diameter is reduced in comparison to the diameter of the remainder of the fiber. Glass fibers of this kind are used in a mixer in which a plurality of the fibers are joined together at the constriction in an attempt to achieve as dense as possible packing. However, an increasing number of fibers will impede a dense packing because the difference between the diameter of the group in the coupling zone and the diameter of the group outside of the coupling zone rapidly increases and the outermost fibers of the coupling zone are subjected to a greater deflection at the coupling zone. Since the bending stability and the drawing stability of an individual fiber is subjected to certain limits particularly at the constriction, the number of fibers which can be packed together is limited.